You’re totally missing the point. HTML is a document language which is intended to be consumed by machines, whereas Markdown is designed to be good for humans to read directly.

If someone writes a Markdown document and it’s littered with complicated directives and tricky bits of syntax which rely on the specific implementation to work, but aren’t readable as plain text, then they’re not really following the spirit of Markdown.

By contrast, since HTML isn’t intended to be read directly, any hacky bit of markup, however awful it makes reading the raw file, is fair game as long as the browser output is pretty.